Sources of azimuthal asymmetries in ionized metal physical vapour deposition processes

The ionized metal physical vapour deposition (IMPVD) process is being developed to produce metal seed layers and diffusion barriers in deep contacts and vias for microelectronics fabrication. An IMPVD reactor is typically an antenna excited system where transmission line effects may produce asymmetric ion fluxes to the target and hence an asymmetric distribution of sputtered metal species in the reactor. A possible result is a non-uniform metal deposition on the wafer. In this paper, a three-dimensional model for an IMPVD reactor is employed to examine the consequences of asymmetric excitation and irregular sputter tracks on species' densities and fluxes. It was found that for typical conditions for Al IMPVD severe asymmetries in electron temperature and electron density profiles produced by a poorly optimized antenna are not reflected in the metal fluxes to the substrate. The metal species have improved symmetry due to charge exchange of the buffer gas ions to the metal and the higher mobility of the metal ions relative to the buffer gas ions. The symmetry and uniformity of the metal species above the wafer significantly improve when increasing the aspect ratio of the plasma region or increasing the pressure due to there being more diffusional transport. However, this improvement is accompanied by a decrease in the magnitude of metal fluxes to the wafer. Irregular sputter tracks combined with rotation of the target were also investigated.